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Dust Control Handbook for Minerals Processing |
Printing Instructions |
Chapter 3: Dust Control Systems
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Why Dust Control?
After dust is formed, control systems are used to reduce dust
emissions. Although installing a dust control system does not assure
total prevention of dust emissions, a well-designed dust control system
can protect workers and often provide other benefits, such as-
- Preventing or reducing risk of dust explosion or fire
- Increasing visibility and reducing probability of accidents
- Preventing unpleasant odors
- Reducing cleanup and maintenance costs
- Reducing equipment wear, especially for components such as bearings
and pulleys on which fine dust can cause a "grinding" effect
and increase wear or abrasion rates
- Increasing worker morale and productivity
- Assuring continuous compliance with existing health regulations
Proper planning, design, installation, operation, and maintenance are
essential for an efficient, cost-effective, and reliable dust control system.
Types of Dust Control Systems
The three basic types of dust control systems currently used in minerals processing operations are-
- Dust collection
- Wet dust suppression
- Airborne dust capture
Dust collection systems use ventilation principles to capture the
dust-filled airstream and carry it away from the source through ductwork
to the collector.
Wet dust suppression techniques use water sprays to wet the material so
that it generates less dust.
Airborne dust capture systems may also use a water-spray technique;
however, airborne dust particles are sprayed with atomized water. When the dust particles collide with the water droplets, agglomerates are
formed. These agglomerates become too heavy to remain airborne and
settle.
Selection of a Dust Control System
The selection of a dust control system is normally made based on the
desired air quality and existing regulations. Dust collection
systems can provide reliable and efficient control over a long period;
however, the capital and operating costs are high. Wet dust
suppression and airborne dust capture systems, while somewhat less
efficient, are less expensive to install and operate but also require
careful selection and planning to be most effective.
The facilities that require dust control should be surveyed in detail
before a dust control system is selected. Emphasis should be placed
on the process, the operating conditions, the characteristics of the
processing equipment, associated dust problems, and toxicity of the
dust. The following is a list of information that may be required:
- Process flow diagram of the facility indicating items such as the
type of material being handled, material flow rates, and the type of equipment
- Major dust emission points and conditions that occur at these points during normal operations
- Desired performance of the system
- Drawings indicating equipment layout
- Retention time of material in bins or stockpiles
- Availability of electrical and other utilities
- Areas requiring freeze protection
Dust Collection System
The dust collection system, also known as the local exhaust ventilation
system, is one of the most effective ways to reduce dust emissions.
A typical dust collection system consists of four major components:
 |
| Components of Dust Collection
System |
- An exhaust hood to capture dust emissions at the source
- Ductwork to transport the captured dust to a dust collector
- A dust collector to remove the dust from the air
- A fan and motor to provide the necessary exhaust volume and energy
Each of these components plays a vital role in proper operation of a
dust collection system, and poor performance of one component can reduce
the effectiveness of the other components. Therefore, careful design
and selection of each component is critical.
The following sections describe the design of exhaust hoods and
ductwork used to remove dust from the process. Chapter 4 discusses dust collectors, fans and motors, which are used to collect the dust for
disposal.
Principles of Airflow
Airflows from a high- to a low-pressure zone due to the pressure
difference. The quantity and the velocity of airflow are related according to the following equation:
Q = AV
Where:
Q =
volume of airflow, ft3/min
V
=
velocity of air, ft/min
A
=
cross-sectional area through which
the air flows, ft2
Air traveling through a duct is acted on simultaneously by two kinds of
pressure:
- Static pressure
- Velocity pressure
Both SP and VP are components of a third kind of pressure:
Static pressure (SP)
SP = Gauge Pressure - Atmospheric Pressure
Static pressure (SP) is a force that compresses or expands the
air. It is used to overcome the frictional resistance of ductwork, as well as the resistance of such obstructions as coils, filter, dust
collectors, and elbows.
SP is the difference between pressure in a dust and that in the
atmosphere. When the SP is above the atmospheric pressure, it has a
positive sign (+); when it is below the atmospheric pressure, it has a
negative sign (-). SP is commonly measured in inches of water.
SP always acts perpendicular to the ductwalls and creates outward
pressure when positive and inward pressure when negative.
Velocity Pressure (VP)
Velocity pressure (VP) is the pressure required to accelerate the air
from rest to a particular velocity. It exists only when air is in motion, always acts in the direction of airflow, and is always positive in
sign. VP is also commonly measured in inches of water.
Note: The relationship, as illustrated, is valid only when g=32.2
ft/s2 (gravitational acceleration constant) and P=0.075 lb/ft3
(air density). For other conditions, a correction factor must be
used.
Total Pressure (TP)
| TP = SP + VP |
 |
| Total Pressure |
Total pressure is the algebraic sum of SP and VP. It is the
pressure required to start and maintain the airflow.
If the velocity of air flowing through a duct increases, part of the
available SP is used to create the additional VP to accelerate the
airflow. Conversely, if the velocity is reduced, a portion of the VP
is converted into SP. These conversions, however, are always
accompanied by a net loss of TP (in other words, te conversion is always
less than 100% efficient).
Vena Contracta
When air enters a suction opening, the airstream gradually contracts a
short distance downstream and, as a result, a portion of the static
pressure is converted into velocity pressure. The plane where the
diameter of the jet is the smallest is known as the vena contracta.
After the vena contracta, the airstream gradually expands to fill the duct
and, consequently, a portion of the velocity pressure is converted into
static pressure. Both of these pressure conversions are accompanied
by losses, which reduce the airflow. The amount of airflow reduction
can be defined by a factor known as the coefficient of entry, "Ce."
 |
| Vena Contracta |
"Ce", Coefficient of Entry
| Ce |
= |
[ |
Actual airflow |
] |
|
| Theorectical airflow |
This represents the percentage of flow that will occur into a given
exhaust hood based on the static pressure developed by the hood. It
is defined as the actual rate of flow caused by a given static pressure
compared to the theoretical flow that would result if there were no losses
due to pressure conversions.
"he", Hood Entry Loss
Related to the Ce is the term "hood entry loss" or
"he." It is defined as the factor representing
the lost in pressure caused by air flowing into a duct. It is
measured in inches of water.
Exhaust Hood
The exhaust hood is the point where dust-filled air enters a dust
collection system. Its importance in a dust collection system cannot
be overestimated. It must capture dust emissions efficiently to
prevent or reduce worker exposure to dusts. The exhaust hood-
- Encloses the dust producing operation
- Captures dust particulates and guides dust-laden air efficiently
Types of Exhaust Hoods
The three general classes of exhaust hoods are-
 |
| Local Hood |
- Side, downdraft and canopy hoods
 |
 |
 |
| Side Hood |
Downdraft Hood |
Canopy Hood |
 |
 |
| Booth Hood |
| |
|
Enclosure Hood |
Local hoods are relatively small structures. They are normally
located close to the point of dust generation and capture the dust before
it escapes. Local hoods are generally efficient and typically used
for processes such as abrasive grinding and woodworking.
Side, downdraft, and canopy hoods are larger versions of local
hoods. They also rely on the concept of preventing dust emissions beyond the control zone. They are typically used for plating
tank exhausts, foundry shakeouts, melting furnaces, etc. These hoods are
generally less efficient than local hoods.
Booth and enclosure hoods isolate the dust generating process from the
workplace and maintain an inward flow of air through all openings to
prevent the escape of dust. These hoods are the most popular type in
minerals processing operations because they are very efficient at minimum
exhaust volumes. They are typically used for areas such as vibrating
or rotating screens, belt conveyors, bucket elevators, and storage bins.
Design of Exhaust Hoods
The design of an exhaust hood requires sufficient knowledge of the
process or operation so that the most effective hood or enclosure (one
requiring minimum exhaust volumes with desired collection efficiency) can
be installed.
The successful design of an exhaust hood depends on-
- Rate of airflow through the hood
- Location of the hood
- Shape of the hood
Of the above three factors, the rate of airflow through the exhaust
hood (that is, the exhaust volume rate) is the most important factor for
all types of hoods. For local, side, downdraft, and canopy hoods,
the location is equally important because the rate of airflow is based on
the relative distance between the hood and the source. The shape of
the exhaust hood is another design consideration. If the hood shape is not
selected properly, considerable static pressure losses may result.
 |
| Air-Induction Approach |
Rate of Airflow - Two approaches used in minerals processing
operations to determine the rate of airflow needed through a hood are-
- Air induction
- Control velocity
Air Induction - The air-induction concept is based n the theory that
when granular material falls through the air each solid particle imparts
some momentum to the surrounding air. Due to this energy transfer, a
stream of air travels with the material. Unless the air is removed
it will escape through all openings upon material impact, carrying the
fine dust particles with it. For adequate control of dust emissions,
the exhaust air volume rate must be equal to or greater than the
air-induction rate.
The air-induction phenomenon is of great significance in calculating
exhaust volumes. The concept can be applied to many transfer points
normally found in minerals processing operations because the calculations
are based on variables such as the material feed rate, its height of free
fall, its size, and its bulk density.
The exhaust volumes are calculated before the exhaust hood is designed
and placed. An approach suggested by Anderson is the most commonly
used in the industry today. It is based on the results of a
comprehensive laboratory study made by Dennis at Harvard School of Public
Health. In its simplified form, it is-
Where:
| Qind = |
volume of induced air, ft3/min |
| Au = |
enclosure open area at upstream end (point where air
is induced into the system
by action of the falling material), ft2 |
| R
= |
rate of material flow rate, ton/h |
| S = |
height of free fall of material, ft |
| D = |
average material diameter, ft |
Due to the approximate nature of the formula, Anderson recommends that-
Qind = QF
Where:
QE = required exhaust volume, ft3/min
The most important parameter in the equation is Au-the
opening through which the air induction occurs. The tighter the
enclosure, the smaller the valve of Au and, hence, the smaller
the exhaust volume.
Although Anderson's approach can be widely applied, it may not be
appropriate for some special operations or situations. When Anderson's approach cannot be applied, the control-velocity approach
should be used.
Control Velocity - In the control-velocity approach, the exhaust hood
is designed before exhaust volumes are computed. This approach is
based on the principle that, by creating sufficient airflow past a dust
source, the dusty air can be directed into an exhaust hood. The air
velocity required to overcome the opposing air currents and capture the
dusty air is known as capture velocity.
 |
| Control-Velocity Approach |
Dallavalle investigated the air-velocity pattern in a space adjoining
an exhaust/suction opening and developed the folowing equation to
determine exhaust volume:
Q = Vx
(10X2 + A)
Where:
Q = exhaust volume,
ft3/min
Vx = centerline
velocity (i.e., capture velocity)
at distance X from hood, ft/min
X = distance outwards
along the hood axis, ft
A = area of hood face
opening, ft2
Capture velocities for some typical operations are provided in the
table on the following page.
Location of the Exhaust Hood- The location of the exhaust hood
is important in achieving maximum dust-capture efficiency at minimum
exhaust volumes. When the control-velocity approach is used, the
location of the hood is critical because exhaust volume varies in relation
to the location and size of the exhaust hood. The location of the
exhaust hood is not as critical when the air-induction approach is used.
 |
Location of Hood Using
Air-Induction Approach |
The air-induction approach requires the hood to be located as far from
the material impact point as possible to-
- Prevent capturing coarse dust particles, which settle quickly
- Capture only fine, predominantly respirable dust
- Reduce unnecessary transport of coarse dust through ductwork and
thus reduce dust settling in horizontal duct runs
- Reduce dust loading (dust concentration) in the exhaust gases
- Minimize subsequent cleaning and disposal of the collected dust
- Prevent capture of valuable products, especially in ore-concentrating operations
The control-velocity approach requires the hood to be located as close to the source as possible to-
- Maximize the hood capture efficiency for a given volume
- Reduce the exhaust volume requirements
- Enclose the source as much as possible
Shape of the Exhaust Hood - Sizable pressure losses may occur if
the shape of the exhaust hood is not designed properly. These
pressure losses are due to the mutual conversion of static and velocity
pressures.
The following points should be considered in selecting the shape of the hood:
- The exhaust hood shape with the highest coefficient of entry value,
Ce, or the lowest hood entry loss factor, he,
should be selected. Various values of Ce and he are
described in the following table:
- Wherever possible, the hood should be flanged to eliminate airflow
from zones containing no contaminants.
This measure can reduce exhaust air volume up to 25%. For most applications, the flange
width should not exceed 6 in.
 |
| Effect of Flanged Opening |
 |
| Reprinted by permission from the Committee on
Industrial Ventilation, Lansing, VI, 18th Edition. |
Ductwork
The ductwork transports the dust captured by the exhaust hood to a dust
collector. Efficient transport of captured dust is necessary for
effective and reliable system operation.
Ductwork Design
Ductwork design includes the selection of duct sizes based on the
velocity necessary to carry the dust to the collector without settling in
the duct. From this information, pressure losses in the duct and
exhaust air volumes can be calculated and used to determine the size and
type of fan, as well s the speed and size of motor.
Before detailed design of the ductwork is begun, the following
information should be available:
- A process flowsheet of the operation indicating-
- Type, size, and speed of the bulk material handling or processing equipment used
- A line diagram of the dust collection system indicating-
- Exhaust hood and exhaust volumes required for each piece of
equipment, each transfer point, and each duct network
- Each branch and section of the main duct, identified either
by number or letter
- A general layout of the facility showing-
-All equipment in the plan and elevations
- The ductwork route and location of the exhaust hood
- Location of the dust collector and the fan
- A preliminary bill of material containing-
- Length of each duct
- Number and type of elbows, transition, and taper pieces, etc.
- Number and size of "y" branches for each branch and
main as identified in the process flowsheet
Proper ductwork design-
- Maintains adequate transport velocities in the duct to prevent particulate settling
- Provides proper air distribution in all branches to maintain designed capture velocities of exhaust hoods
- Minimizes pressure losses, wear, and abrasion of ductwork thus reducing operating costs
Transport Velocities - To prevent dust form settling and
blocking the ductwork, transport velocities should range from 3,500 to
4,000 ft/min for most industrial dust (such as granite, silica flour,
limestone, coal, asbestos, and clay) and from 4,000 to 5,000 ft/min for
heavy or moist dust, such as lead, cement, and quick lime. The table
describes minimum transport velocities for different characteristics of
dust.
| Material |
Minimum Design Velocity (fpm) |
|
|
| Very fine, light dusts |
2,000 |
| Fine, dry dusts and powders |
3,000 |
| Average industrial dusts |
3,500 |
| Coarse dusts |
4,000 - 4,500 |
| Heavy or moist dust loading |
4,500 and up |
Note: The minimum transport velocity indicated in the table is
for guidance only. The design velocity should be estimated by
including a safety factor in the above minimum velocities.
Estimation of safety factors should consider-
- Material buildup
- Duct damage
- Corrosion of ductwork
- Duct leakage
Distribution of Airflow - Proper airflow distribution in each
branch is necessary to maintain adequate capture and transport velocities
in the system. If air is not properly distributed in a
multiple-branch dust collection system, a natural balance will take
place. For example, the exhaust volume will be determined by the
resistance of the available flow paths, and the branch with the least
resistance will carry the most volume. As a result, the desired
airflow may not be achieved in each branch.
The system should be balanced to ensure desired airflow
distribution. In other words, all branches entering a junction must
have equal static pressures at the designed flow. Two methods
available to balance the system are-
- Air balance without blast gates
- Air balance with blast gates
Air Balance Without Blast Gates - This method, often called the static
pressure balance method, provides a way to achieve the desired airflow (a
balanced system) without the use of dampers or blast gates.
Calculation begins at the branch of greatest resistance and proceeds from
branch to main, through each section of main, and to the fan. At
each junction of two airstreams, the static pressure necessary to achieve
desired flow in both streams is matched and, thus, branches are brought
into "balance." The static pressures can be balanced at
the desired rate of flow by choosing appropriate sizes of ducts, elbow
radii, etc.
Air Balance With Blast Gates - This method uses blast gates to achieve
the desired airflow at each hood. Calculation begins at the branch of greatest resistance, and pressure drops are calculated through the
branch and through the various sections of the main to the fan. No attempt is made to balance the static pressure in the joining
airstreams. The joining branches are merely sized to provide the
desired transport velocities.
Note: Choosing the branch of greatest resistance is critical in
this method. If the choice is incorrect, any branch or branches
having a higher resistance will fail to draw the desired volume even when
their blast gates are wide open. To prevent this error, all branches
that could possibly give the greatest resistance must be checked.
Selection of Balancing Method - Both of the above approaches are
common. However, air balance without blast gates normally is selected for processes where highly toxic materials are exhausted so that
possible tampering with blast gates will not affect airflow. Air
balance with blast gates is selected when exhaust volumes cannot be
properly estimated or the system requires some flexibility in varying
exhaust volumes.
Note: In the air balance without blast gates method, although
calculations are time consuming during the design stage, airflow in the field need not be measured and balanced. In the air balance with
blast gates method, the design calculations are fast, but considerable efforts are required in the field to measure and adjust the blast gates to
achieve the balance.
Irrespective of the method selected, additional hoods should not be
added once a multiple hood layout is completed and balanced because
they may alter the airflow and make some other hoods totally
ineffective. A comparison of both balancing methods is provided in
the table on the following page.
Pressure Losses - Pressure losses occur when air travels in a
duct. To overcome these pressure losses, power is supplied by the
fan and motor. The higher the pressure losses, the greater the motor
horsepower requirements.
Pressure losses in a dust collection system occur due to the following:
 |
| Hood-Entry Losses |
- Hood entry
- Special duct fittings
- Duct friction
- Air-cleaning devices
Hood Entry Losses - A loss in pressure occurs when air enters a suction
or hood opening. This loss is indicated by the coefficient of entry
for the hood (Ce). Several examples of entry coefficients
are illustrated.
Losses from Special Dust Fittings - When air travels through the
various duct fittings, such as elbows, "y" branches,
enlargements, or contractions (tapers), pressure losses occur.
Pressure loss across these fittings is expressed in one of two ways"
- As a fraction of the velocity pressure
- In terms of equivalent feet of straight duct (of the same diameter)
that will produce the same pressure loss as the fitting
Duct-Friction Losses - When air travels in a straight run of duct,
pressure losses occur due to the friction between the duct walls and
air. Many charts and graphs are available that give friction losses
in straight ducts. However, most of them are based on new, clean
ducts. The following chart, which allows for a typical amount of
roughness, plots four quantities. If any two quantities are known,
the other two can be read directly from the chart.
 |
| Duct Friction Losses |
| Reprinted by permission from the Committee on Industrial Ventilation, Lansing,
MI, 18th Edition. |
Losses from Air-Cleaning Devices - In addition to pressure losses in
the ductwork, the losses in the dust collector must also be known. Although the pressure drop for dust collectors varies widely, data are
usually available from manufacturers. More information on dust collectors can be obtained from chapter 4.
Points to Note in Ductwork Design/Layout - To minimize pressure losses,
the Industrial Ventilation Manual recommends the following guidelines for
ductwork design:
- All branches should enter the main at 30º angle; wherever possible,
the velocity should match that of the incoming gas stream.
- Duct size changes should be kept to a minimum. If needed, they should be gradual.
- Wherever possible, a circular duct should be used instead of a
rectangular duct to maintain uniform velocity distribution and prevent settling of material in the ductwork.
- Wherever possible, flanges should be provided to minimize hood entry losses.
- The centerline radii of all elbows should be at least twice the diameter of the duct.
The illustrations below provide examples of ductwork design.
 
  |
| Reprinted by permission from the Committee on Industrial
Ventilation, Lansing, MI, 18th Edition. |
Example Problem
This example, which illustrates the balancing of ductwork, is provided
to aid understanding of the detailed ductwork design procedure. In
the example, the balancing of ductwork is based on the air balance without
blast gate method, and the resistances are based on the equivalent foot
basis. This approach is one of several available for balancing
ductwork; however, an understanding of this approach should facilitate
understanding of other approaches. Information on other approaches
can be obtained from the sources provided in the references.
The Problem
Design a dust collection system for an industrial sand-handling
facility.
Information provided
- Process flowsheet and schematic of the dust collection system
- Minimum transport velocity = 3,500 fpm
- Necessary exhaust volumes
- Description and materials
 |
| Process Flowsheet |
Details of Operation
|
| Description |
Number |
Minimum Exhaust Volume (cfm) |
|
|
|
| Bag Machine Hood |
1 |
800 |
| Conveyor Transfer Point |
2 |
300 |
| Bag House Dust Collector |
3 |
- |
| Fan |
4 |
|
Description of
Material
|
| No. of Branch or Main |
Airflow Required (cfm) |
Straight Run (ft) |
Number of Elbows |
Number of Entries |
|
|
|
|
|
| 1-b |
800 |
30 |
2-90º |
-- |
| 2-b |
300 |
35 |
3-90º |
-- |
| b-c |
1,100 |
50 |
-- |
1 |
| c-d |
1,100 |
0 |
1-90º |
-- |
| d-e |
1,100 |
30 |
-- |
-- |
After gathering the above information you can start to fill in the
worksheet table:
| Column |
Entry |
Explanation
|
|
|
|
| 1 |
1-b |
Section of system to be worked on from
bag hood to Y junction |
| 2 |
6.5 in. |
Based on minimum transport velocity and
minimum air volume required using Q=VA formula A=0.27 ft2,
this gives a duct size of 6.5 in. |
|
0.27 |
Duct Area |
| 4 |
800 |
Air volume required, as calculated from
Anderson, or others, or determined by past experience or testing |
| 6 |
3500 |
Air velocity determined by Q/A = V |
| 7 |
30 |
Straight runs measured from prints or
on site |
| 8 |
2 |
Number of elbows determined from
schematic installation |
| 9 |
14 |
Taken from table ___ for an elbow with
a centerline radius of 2.0 times the duct diameter. This
number
is multiplied times the number of elbows. |
| 10 |
44 |
Sum of straight rum length (col.7) and
equivalent length (col.9) |
| 11 |
2.7 |
Frictional losses read directly from
Figure ___, look at duct diameter vs. duct velocity and read
frictional loss per 100 ft. of straight total duct length |
| 12 |
1.19 |
Total duct length (col.10) x frictional
loss per 100 ft. of duct (col.11) divided by 100 |
| 13 |
0.77 |
Velocity pressure of air in duct, VP =
(V/P)2 |
| 14 |
0.5 |
Entry loss for hood (given) |
| 15 |
1.5 |
The total exhaust hood loss. This
represents the amount of energy required to get the air to flow into the hood (1.0 VP) plus the
specific hood entry loss (col. 14). |
| 16 |
1.16 |
Product of cols. 13 and 15 |
| 17 |
1.16 |
The governing static pressure in branch
1-b (col. 12 + col. 16). |
Repeat this procedure for each branch circuit, when you reach a
junction of two branch circuits the balanced pressure method requires that
governing static pressure at the junction be within 5% of one
another. If this is not the case, as in our example, then design
parameters must be altered to achieve balance Several things might be
done:
- lower the resistance in branch 2-b by increasing duct size, or
reducing air volume
- increase the resistance in branch 1-b by decreasing duct size or
increasing air volume
Engineering and economic judgment should be used to make this decision;
for instance, whether you can use additional air volume or your fan cannot
handle the static pressure may dictate the way in which you choose to
balance your system. In the example we chose to increase the resistance in branch 1-b to achieve balance. This step may require
several trial and error attempts until you become familiar with the
process.
Continue filling in the work sheet using the governing static pressure
column to keep a running total of pressure (note that pressures in series circuits are additive; in parallel circuits they are not). The
governing pressure in that branch is used.
Finally, as the air exits the exhaust stack of the fan, the velocity
pressure of the air is converted back to static pressure and results in
the recovery of that energy which is subtracted from the governing static
pressure of the system.
| WORKSHEET |
| Example Problem |
 |
Wet Dust Suppression System
Wet dust suppression systems wet the entire product stream so that it
generates less dust. This also prevents dust from becoming
airborne. Effective wetting of the material can be achieved by-
 |
| Contact Angle of a Water Droplet |
- Static Spreading - The material is wetted while
stationary. The diameter and contact angle of water droplets are
important factors in static spreading.
The surface coverage can be increased by reducing either the
droplet diameter or its contact angle.
- Dynamic Spreading - The material is wetted while
moving. The surface tension of the liquid, the droplet diameter,
the material size, and the droplet impact velocity are important
variables in dynamic spreading.
The surface coverage can be increased either by reducing the
surface tension or by increasing the impact velocity.
One of these two water spreading methods can be emphasized at the
expense of the other, depending on the needs of the system. For
example, both static and dynamic spreading of a droplet can be increased
by reducing the surface tension and thus decreasing the droplet
diameter. However, the impact velocity of smaller droplets decreases
faster due to frictional drag and less momentum, which, in turn, reduces
dynamic spreading. An optimum droplet diameter for maximum material
surface coverage must therefore be determined.
Factors Affecting Surface Wetting
Droplet Size
Surface wetting can be increased by reducing the droplet diameter and
increasing the number of droplets. This can be achieved by reducing
the surface tension/contact angle. The surface tension of pure water
is 72.6 dyne/cm. It can be reduced from 72.6 to 28 dyne/cm by adding
minute quantities of surfactants. This reduction in surface tension
(or contact angle) results in-
- Reduced droplet diameter
- An increase in the number of droplets
- A decrease in the contact angle
Impact Velocity
Surface wetting can be increased by increasing the impact
velocity.
Impact Velocity can be increased by increasing the
system's operating pressure.
Note: A droplet normally travels through turbulent air before it
impacts on the material surface. Due to the frictional drag of the
turbulent air, the impact velocity of the droplet is less than its
discharge velocity from the nozzle. Moreover, small droplets lose
velocity faster than large ones. To cover the greatest surface area,
the best impact velocity for a given droplet diameter must be determined
for each operation.
Types of Wet Dust Suppression Systems
Wet suppression systems fall into three categories:
- Plain Water Sprays - This method uses plain water to wet the
material. However, it is difficult to wet most surfaces with
plain water due to its high surface tension.
- Water Sprays with Surfactant - This method uses surfactants
to lower the surface tension of water. The droplets spread
further and penetrate deeper into the material pile.
- Foam - Water and a special blend of surfactant make the foam. The
foam increases the surface area per unit volume, which increases wetting efficiency.
Advantages and Disadvantages
|
| Advantages |
Disadvantages |
|
|
| Plain Water Sprays |
- It is probably the least expensive method
of dust control.
|
- Water sprays cannot be used for
products that cannot tolerate excessive moisture.
|
- The system is simple to design and
operate
|
- Water sprays cannot be used when
temperatures fall below freezing.
|
- A limited carryover effect at subsequent transfer points is possible
|
- Usually, dust control efficiency is low, unless large quantities of water are used.
|
- When good mixing of water and material can be achieved, dust
generation can be reduced effectively.
|
- Freeze protection of all hardware is necessary
|
- Enclosure tightness is not essential.
|
- Careful application at transfer points that precede a screen
is required to prevent blinding.
|
|
Water Sprays With
Surfactants |
- This method is used when surfactants are tolerated but
excessive moisture is not acceptable.
|
- Capital and operating costs are higher than water-spray systems.
|
- In some cases, dust control efficiency is higher than with plain water sprays.
|
- Careful application at transfer points that precede a screen is required to prevent blinding.
|
- Equivalent efficiency is possible with less water.
|
- Equipment such as the pump and proportioning equipment used to
meter the flow of surfactant require maintenance.
|
| |
- Freeze protection of all hardware is necessary
|
|
Foam |
- When good mixing of foam and product stream can be achieved,
dust control efficiency is greater than water with surfactants.
|
- Operating costs are higher than with finely atomized water-spray systems.
|
- Moisture addition is usually less than 0.1% of the material weight.
|
- The product is contaminated with surfactants.
|
| |
- Careful application at transfer points that precede a screen
is required to prevent blinding.
|
Airborne Dust Capture Systems
In this approach, very fine water droplets are sprayed into the dust
after it is airborne. When the water droplets and dust particles
collide, agglomerates are formed. When these agglomerates become too
heavy to remain airborne, they settle.
 |
 |
| Collision Between Dust Particle and Water Droplet |
Coalescence or Adhesion Between Dust Particle and Water Droplet |
Factors Affecting Collision
The collision between dust particles and water droplets occurs due to
the following three factors:
- Impaction/interception
- Droplet size/particle size
- Electrostatic forces
Impaction/Interception
When a dust particle approaches a water droplet, the airflow may sweep
the particle around the droplet or, depending on its size, trajectory, and
velocity, the dust particle may strike the droplet directly, or barely
graze the droplet, forming an aggregate.
 |
| Particle Trajectories Around a Water Droplet |
Droplet Size/Particle Size
Droplets and particles that are similar in size have the best chance of
colliding. Droplets smaller than dust particles or vice versa may
never collide but just be swept around one another.
 |
Effect of Droplet Size
Schowengerdt and Brown |
Electrostatic Forces
The presence of an electrical charge on a droplet affects the path of a
particle around the droplet. When particles have an opposite or
neutral charge, collision efficiency is increased.
 |
 |
| Oppositely Charged Droplet and Particle Attract
Each Other |
Similarly Charged Droplet and Particle Oppose
Each Other |
Types of Airborne Dust Capture Systems
Airborne dust capture systems can be simple or quite complex.
Basically, they fall into two broad groups:
- Finely atomized water sprays
- Electrostatically charged fogs
Finely Atomized Water Sprays
Finely atomized water sprays are normally used at transfer points
without excessive turbulence or when the velocity of dust dispersion is
less than 200 ft/min. The optimum droplet size, water usage,
relative velocity, and number and location of nozzles depend on the conditions at individual transfer points.
Electrostatically Charged Fogs
Electrostatically charged fog uses charged water droplets to attract
dust particles, which increases collision. The atomized water droplets are charged by induction or direct charging.
Design of a Water-Spray System
The spray nozzle is the heart of a water-spray system. Therefore,
the physical characteristics of the spray are critical. Factors such
as droplet size distribution and velocity, spray pattern and angle, and
water flow rate and pressure all vary depending on the nozzle
selected. Following is a general discussion of these important
factors:
- Droplet Size- The nozzle's droplet size distribution is the
most important variable for proper dust control. The droplet
size decreases as the operating pressure increases. Information
about the droplet size data at various operating pressures can be
obtained from the nozzle manufacturer. For wet dust suppression
systems, coarse droplets (200-500 µm) are recommended. For
airborne dust capture systems, very fine droplets (10-150 µm) may be required. The fine
droplets usually are generated by fogging nozzles,
which may use either compressed air or high-pressure water to atomize
water in the desired droplet range.
- Droplet Velocity - Normally, higher droplet velocities are
desirable for both types of dust control through water sprays. Information on the
droplet velocity can be obtained from the nozzle manufacturer.
- Spray Pattern - Nozzles are categorized by the spray patterns
they produce:
- Solid-cone nozzles product droplets that maintain a high velocity
over a distance. They are useful for providing a high-velocity spray
when the nozzle is located distant from the area where dust control is
desired.
- Hollow-cone nozzles produce a spray patter in the form of circular
ring. Droplet range is normally smaller than the other types of
nozzles. They are useful for operations where dust is widely
dispersed.
- Flat-spray nozzles produce relatively large droplets that are
delivered at a high pressure. These nozzles are normally useful for
wet dust suppression systems (i.e., preventive type systems).
-Fogging nozzles produce a very fine mist (a droplet size distribution
ranging from submicron to micron). They are useful for airborne dust
control systems.
 |
 |
 |
 |
Solid-Cone
Noozle |
Hollow-Cone
Nozzle |
Flat-Spray
Noozle |
Fogging
Noozle |
- Spray Angle - Each nozzle has a jet spray angle. The size
of this angle is normally available from the manufacturer. A
knowledge of spray angle and spray pattern is essential to determine the
area of coverage and, therefore, the total number of nozzles needed.
- Flow Rate - The flow rate of water through a nozzle depends
on the operating pressure. The flow rate and operating pressure
are related as follows:
 |
| where K = nozzle constant |
 |
 |
| Spray Angle |
| Water Flow/Pressure |
| From Bureau of Mines open File Report 145-82,
Guide Book for Dust Control in Underground Mining, December 1981. |
A knowledge of the water flow rate through the nozzle is necessary to
determine the percentage of moisture added to the material stream.
The following factors should be considered in selecting the nozzle location:
- It should be readily accessible for maintenance.
- It should not be in the path of flying material.
- For wet dust suppression systems, nozzles should be upstream
of the transfer point where dust emissions are being created.
Care should be taken to locate nozzles for best mixing of material and
water. For airborne dust capture, nozzles should be located to
provide maximum time for the water droplets to interact with
the airborne dust.
| Advantages and Disadvantages |
| Advantages |
Disadvantages |
|
|
| Finely Atomized Water Sprays |
- Water requirements are low-typically 5 to 20 gal/h per
nozzle.
|
- Tight enclosures are needed for effective system
operation.
|
- Moisture addition to the product is quite
low-typically less than 0.1% of the material weight.
|
- The system may not be effective either in highly
turbulent environments or when the dust dispersion rate is more than 200 ft/min.
|
- The material is not chemically contaminated.
|
- Requires good droplet to particle size match for
effective control.
|
- The system can be economical.
|
|
| Electrostatically Charged
Fogs |
- Electrostatic fogs can be effective if the dust cloud
carries predominantly positive or negative charges.
|
- These systems are not recommended for underground coal
mines or other gassy applications where explosions can be triggered by sparks.
|
- The material does not become chemically contaminated.
|
|
- Moisture addition to the product is generally less
than 0.5% by weight.
|
- These systems require high-voltage equipment.
|
| |
- Maintenance of electrical insulation is critical for
safe working conditions.
|
Water Flow and Compressed Airflow Rates
Once the nozzle is selected, its spray pattern and area of coverage can
be used to determine water flow rate and/or compressed airflow rates and
pressure requirements. This information is normally published by the
nozzle manufacturer. These must be carefully coordinated with the
maximum allowable water usage. Water flow rates will be highly
variable depending on the size and type of material, the type of
machinery, and the throughput of material.
Piping Design
The piping should be designed so that each nozzle receives water or
compressed air at specified flow rates and pressures. Drains must be
provided at the lowest point in each subcircuit of the piping system to
flush the air and water liens in winter months. Heat tapes and
insulation must also be provided at locations where the temperature may
drop below 32º F. The heat tracing tape should be able to provide
approximately 4 watts per linear foot for water pipes up to 2 in. in
diameter. The pump and other hardware, such as valves and gauges,
should also be heat traced and insulated to prevent freezing during winter
months.
Instruments
Pressure and flow gauges are recommended to monitor system
performance. These instruments should be located as close to the
point of application as possible. Liquid-filled pressure gauges and
rotameter-type flowmeters are satisfactory and quite inexpensive.
For situations where it is desirable to activate wet suppression
systems only when the material is flowing (for example, if the belt
conveyor is running empty, water sprays need not be on), a
solenoid-activated valve may be installed in the water line. The solenoid
can be activated by instruments such as the level controller or
flow sensor. This measure will reduce water usage, reduce
maintenance and cleanup, and reduce or prevent freezeup problems.
Pump and Compressor Selection
An appropriate pump and compressor (where applicable) should be
selected once the airflow and waterflow rates and pressure are determined.
An approximate method of determining the proper pumping energy for
water at 40:1 efficiency is-
| Pump HP = |
1.40 |
x p x q |
|
| 10,000 |
Where:
p = pressure drop in water lines, psig
q
= water flow rate, gal/min
An approximate method of selecting a compressor is by assuming that-
One horsepower of compressor can provide approximately 4 std ft3/min
of compressed air, at 100 psig pressure.
|
| |
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